Go Modules

[pda]

Use Go Modules to manage dependencies, instead of committing them all in vendor/ and tracking them with vendor.json.

This was basically done by:

go mod init github.com/puma/puma-dev
go mod download
go build ./cmd/puma-dev
go test ./...

The tests pass with the resulting go.mod on my Linux environment when combined with the Linux fix in #225

[pda]

I've rebased this onto the now-merged #227 so that modules work in CI, which should give some confidence that this change is 👌